Large-sized vehicles such as trucks frequently incorporate a braking force retainer device configured to apply brakes automatically to retain the vehicles in unmoved states. As an example of the braking force retainer device, a brake fluid pressure retainer device capable of being incorporated into a fluid pressure braking device may be given. The brake fluid pressure retainer device is configured to automatically maintain a brake fluid pressure applied to a braking device for wheels persistently when the vehicle is stopped by applying brakes to the wheels. Accordingly, a status of the brakes being applied to the wheels may be retained without a driver depressing a brake pedal.
The brake fluid pressure retainer device includes an electromagnetic open-close valve disposed on a brake fluid supply pipe. The electromagnetic open-close valve is disposed in a middle of the brake fluid supply pipe connected between the brake pedal and the braking device (e.g., a caliper) for the wheels. Hence, when the driver depresses the brake pedal, (i.e., when the brakes are applied), a brake fluid passes through the electromagnetic open-close valve to flow toward a braking device side of the wheels, which may raise the brake fluid pressure inside the braking device to increase braking force. When the driver eases the depressed brake pedal, (i.e., when the applied brakes are released), the brake fluid passes through the electromagnetic open-close valve to flow back to a brake pedal side, which may lower the brake fluid pressure inside the braking device to decrease the braking force.
To maintain the brake fluid, the electromagnetic open-close valve is closed while the brake fluid pressure inside the braking device for the wheels is high. Accordingly, the brakes being applied to the wheels may be maintained. That is, the electromagnetic open-close valve may air-tightly close in the above fashion to maintain a high pressure brake fluid inside the braking device. As the electromagnetic open-close valve, a so-called “normally-open” electromagnetic open-close valve configured to be closed when a current is supplied to a solenoid may frequently be used.
When the brake fluid pressure inside the braking device is maintained by air-tightly closing the electromagnetic open-close valve, the electromagnetic open-close valve may need to be closed completely.
Otherwise, the brake fluid may leak from the brake pedal side, thereby failing to maintain the brake fluid pressure inside the braking device. Accordingly, the electromagnetic open-close valve may need to have a structure capable of being air-tightly closed completely without allowing the brake fluid pressure to cause leakage from the electromagnetic open-close valve. In order to reduce a size of the electromagnetic open-close valve, an electromagnetic force (corresponding to a pressing force to press a valve against a valve seat) generated from the solenoid may need to be reduced. Accordingly, the electromagnetic open-close valve is configured to have a reduced cross-section of an opening part of a brake fluid passage, such that the electromagnetic open-close valve may be completely closed by a small-sized solenoid (i.e., a small electromagnetic force).
However, when the size of the cross-section of the opening part of the brake fluid passage is reduced, the reduced part of the cross-section of the opening part may generate a brake fluid pressure to serve as a resistance against the brake fluid that passes through the opening part of the brake fluid passage inside the electromagnetic open-close valve. Accordingly, the brake fluid will not flow rapidly inside the electromagnetic open-close valve, thereby degrading responsiveness of the brakes. In order to overcome the degraded responsiveness of the brakes, there is proposed a technology to dispose a check valve (non-return valve) in parallel with the electromagnetic open-close valve (see Patent Document 1). The check valve is configured to allow the brake fluid to flow from the brake pedal side to the braking device side, and to prevent the brake fluid from flowing from the braking device side to the brake pedal side.